Infectious Disease and Microbiology – Haemophilus influenzae

Overview

Haemophilus influenzae is a small Gram-negative coccobacillus that can cause both invasive and mucosal disease. Clinically, it is useful to distinguish encapsulated strains, especially serotype b (Hib), from nonencapsulated or nontypeable strains.

Before widespread Hib vaccination, H. influenzae type b was a major cause of meningitis, epiglottitis, bacteremia, cellulitis, and septic arthritis in children. Vaccination has dramatically reduced these invasive childhood infections.

Microbiologic Characteristics

H. influenzae is:

• A small Gram-negative coccobacillus

• Facultatively anaerobic

• Fastidious in culture

• Capable of existing as encapsulated or nonencapsulated strains

A classic laboratory feature is its requirement for:

Factor X = hemin

and

Factor V = NAD

for growth.

Culture Characteristics

H. influenzae grows well on:

Chocolate agar

because heating of blood releases the required X and V factors.

It may also demonstrate the satellitism phenomenon when growing near organisms such as Staphylococcus aureus, which supply growth factors.

Incubation Period

For invasive disease such as meningitis, the incubation period is not precisely established, but the source supports approximately:

2–4 days

Epidemiology

H. influenzae occurs worldwide.

The epidemiology changed markedly after introduction of conjugate vaccines against Hib.

Routine Hib immunization has produced a dramatic reduction in invasive serotype b disease in vaccinated populations.

Encapsulated H. influenzae Type b

Importance of the Capsule

The polysaccharide capsule, particularly the serotype b capsule, is a major virulence factor.

Hib can invade the bloodstream and disseminate to normally sterile sites, causing severe disease especially in young children.

Invasive Hib Disease in Children

Classically, Hib causes:

• Meningitis

• Epiglottitis

• Cellulitis

• Septic arthritis

• Bacteremia

These infections are often associated with bloodstream invasion.

Hib Meningitis

Before widespread vaccination, Hib was one of the major causes of bacterial meningitis in young children.

Clinical manifestations may include:

• Fever

• Irritability

• Lethargy

• Vomiting

• Neck stiffness

• Altered mental status

• Seizures in severe disease

This presentation is now much less common in appropriately vaccinated populations.

Epiglottitis

Hib is classically associated with acute epiglottitis, particularly in unvaccinated children.

Typical findings include:

• Abrupt fever

• Severe sore throat

• Dysphagia

• Drooling

• Muffled voice

• Inspiratory stridor

• Respiratory distress

A child may sit in a tripod position to maximize airway patency.

Epiglottitis – Airway Emergency

The major danger of epiglottitis is:

Rapid upper-airway obstruction

Therefore, airway management takes priority over attempts to directly examine the throat in a patient with severe suspected epiglottitis.

Nontypeable H. influenzae

Overview

Nonencapsulated strains, commonly called nontypeable H. influenzae (NTHi), more often cause localized mucosal respiratory infections.

These infections are particularly common in older children and adults.

Otitis Media

Nontypeable H. influenzae is an important cause of:

Acute otitis media

especially in children.

Sinusitis

Nontypeable strains also commonly contribute to:

Acute bacterial sinusitis

often alongside organisms such as Streptococcus pneumoniae and Moraxella catarrhalis.

Chronic Bronchitis and COPD Exacerbation

In adults, particularly those with chronic airway disease, nontypeable H. influenzae may cause:

• Acute exacerbations of chronic bronchitis

• COPD exacerbations

• Lower respiratory tract infection

Pneumonia

H. influenzae may cause pneumonia, particularly in:

• Older adults

• Patients with chronic lung disease

• Immunocompromised individuals

Nontypeable strains are particularly important in adult respiratory infections.

Bacteremia

Although invasive bloodstream infection is classically associated with encapsulated strains, bacteremia can occasionally occur with nonencapsulated strains as well.

Severe Infection in Asplenic Patients

Patients with absent or impaired splenic function are at increased risk for severe infections from encapsulated organisms.

Thus, H. influenzae can produce:

Rapidly progressive sepsis

in patients with:

• Anatomic asplenia

• Functional asplenia

The clinical course can be fulminant.

Epididymitis and Orchitis

The source also lists:

• Epididymitis

• Orchitis

as uncommon manifestations of H. influenzae infection.

Diagnosis

The source describes antigen detection methods including:

• Coagglutination

• Counterimmunoelectrophoresis

• Latex agglutination

These techniques can detect bacterial antigen in secretions or sterile body fluids.

Modern Diagnostic Approach

Depending on the clinical syndrome, diagnosis may also include:

• Culture

• Blood cultures

• CSF culture

• Respiratory specimen culture

• PCR or other molecular testing

For invasive disease, culture and molecular methods are generally more informative than older antigen-detection techniques alone.

Treatment

The source lists:

Amoxicillin–clavulanate

or

Second- or third-generation cephalosporins

as treatment options.

Selection depends on the site and severity of infection.

Invasive Disease

For serious invasive infections such as meningitis, a third-generation cephalosporin, such as ceftriaxone or cefotaxime, is typically an important therapeutic choice.

β-lactamase production and other resistance mechanisms can make plain ampicillin or amoxicillin unreliable without susceptibility information.

Additional Treatment

The source lists:

• Trimethoprim–sulfamethoxazole

• Fluoroquinolones

• Azithromycin

• Aztreonam

• Imipenem

• Meropenem

Choice should be guided by the infection site, severity, patient factors, and susceptibility results.

β-Lactamase Production

Some H. influenzae strains produce β-lactamase, resulting in resistance to ampicillin and amoxicillin.

Therefore:

Amoxicillin alone may fail

whereas:

Amoxicillin–clavulanate

can overcome many β-lactamase-producing strains.

Prevention

Hib Conjugate Vaccine

The most important preventive measure is:

Hib conjugate vaccination

The vaccine contains capsular polysaccharide linked to a protein carrier, allowing an effective immune response in young children.

It is highly effective and has dramatically reduced invasive Hib disease.

Age for Vaccination

The source notes effective vaccination in children older than:

2 months

which corresponds to the age at which routine infant Hib immunization programs begin in many countries.

Postexposure Prophylaxis

Close contacts of a patient with invasive Hib disease may require antimicrobial prophylaxis under appropriate public-health circumstances.

The classic drug is:

Rifampin

The source also mentions ciprofloxacin as a protective measure.

Who May Need Prophylaxis?

Postexposure prophylaxis is particularly considered for selected:

• Household contacts

• Childcare contacts

• Individuals in environments containing incompletely vaccinated or vulnerable young children

Public-health recommendations should guide who receives prophylaxis.

High-Yield Clinical Pattern – Hib

Unvaccinated young child

• ●

Fever

• ●

Meningitis, epiglottitis, cellulitis, or septic arthritis

• ●

Bacteremia

→ Think Haemophilus influenzae type b

High-Yield Clinical Pattern – Nontypeable H. influenzae

Adult with chronic lung disease

• ●

COPD/chronic bronchitis exacerbation

or

Child with otitis media or sinusitis

→ Think nontypeable H. influenzae

Classic Laboratory Pattern

Small Gram-negative coccobacillus

• ●

Requires factor X and factor V

• ●

Grows on chocolate agar

→ Think Haemophilus influenzae

Hib vs. Nontypeable H. influenzae

Hib:

Encapsulated → invasive disease → meningitis, epiglottitis, bacteremia, septic arthritis

Nontypeable strains:

No capsule → mucosal respiratory disease → otitis, sinusitis, bronchitis/COPD exacerbation, pneumonia

Exam Essentials

Organism: Haemophilus influenzae

Type: Gram-negative coccobacillus

Growth requirements: Factors X and V

Culture medium: Chocolate agar

Major virulence factor of Hib: Polysaccharide capsule

Important serotype: Type b

Incubation for invasive disease: Approximately 2–4 days

Hib infections: Meningitis, epiglottitis, bacteremia, cellulitis, septic arthritis

Nontypeable infections: Otitis media, sinusitis, bronchitis/COPD exacerbation, pneumonia

High-risk group for fulminant sepsis: Asplenic patients

Diagnosis: Culture, molecular testing; antigen detection historically used

Treatment: Amoxicillin–clavulanate for appropriate mucosal disease; third-generation cephalosporins for serious invasive disease

Resistance mechanism: β-lactamase production

Prevention: Hib conjugate vaccine

Postexposure prophylaxis: Rifampin for selected close contacts

Key clinical pearl: Haemophilus influenzae type b is an encapsulated invasive pathogen classically associated with meningitis and epiglottitis in unvaccinated children, whereas nontypeable strains primarily cause otitis media, sinusitis, COPD exacerbations, and pneumonia. The organism requires factors X and V and classically grows on chocolate agar.